#!/usr/bin/env python3
import numpy as np

th = -3.15

a=[[np.cos(th),-np.sin(th)],[np.sin(th),np.cos(th)]]

print(a)

r_OtoM = np.arccos(a[0][0])
if (a[1][0] < 0):
    r_OtoM = -r_OtoM
print(r_OtoM)